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This is a fascinating question that has kept legions of scientists arguing for half a century! If Adriana hadn't put me on the spot I don't think I'd have the courage to try answering it, but... here goes. Two caveats: First, I'm going to be answering this more from a systems point of view than a scholarly one. Systems theory and complexity theory predict that complex, self-organizing systems will follow certain forms that are fractal and scale independent, with competition and cooperation within and between subsystems and system levels as the recurring motif. In some cases, I'm choosing between competing narratives or scientific points of view simply because they fit the systems schema better. And second I'm going to be wildly oversimplifying, because even then this is going to be too long. So please understand that nothing I say here will stand up to serious critical scrutiny. I'm trying to get the overview right, even if it's at the expense of some important details. Okay, with that said, the simplest answer is that: Natural selection happens simultaneously at '''every possible level', wherever there is a selective advantage, whether that is for a single allele (version of a particular gene), a sequence of genes and other genetic material, a genotype, a population, a species, a culture, an ecosystem, or even a symbiotic relationship.'' "Selfish" genes In rare cases, a single allele can "cheat" on other genes if it can somehow skew the process of meiosis or recombination to its advantage. However, doing so can reduce the fitness of the whole organism, which puts a limit on how successful this strategy can be. If an allele can guarantee that all descendants of that organism will carry that allele, it does little good if in the long run the allele also causes the organism to be less fit and leave no descendants. Gene selection An allele that gives most individuals an advantage in survival and reproduction will usually spread through the population. For most of us, that's what "natural selection" really means. But note that it has to be an allele that works well with other genes in the population genome. There's no such thing as "fitness" without reference to the potential mates and offspring for the individual carrying the gene. Organism selection All the genes in an individual organism's genome are collectively subject to selection at the organism level. At this level, assuming none of the genes carry crippling or lethal defects, what matters most is how well this set of genes (alleles) works together to create an organism that is fit to survive and reproduce in its particular environment. However, the limit on this is that its superior fitness may come at the expense of its breeding population, species, or ecosystem, which again puts a cap on the degree of competition at this level. An extreme example: a gene for high and deadly male aggression in an otherwise relatively pacific species might allow a single individual to father 100% of the next generation in the local breeding population. But then all of the sons who inherit the gene will expend themselves in bloody battles against each other, greatly weakening that population or even killing it off if the species depends on the males to guard against predators. Population selection The mix of genomes in a single breeding population can also compete against other populations in complex ways. Imagine that we're talking about a complex set of traits for alertness, concentration, and sleep cycles in a social species where bands fight with other bands for territory. There may be dozens or hundreds of genes involved, but lets say that A is a gene cluster that gives rise to individuals who rise early, go to sleep early, concentrate very well, have low aggression, and resist being distracted by random stimuli. These are good, peaceful, productive workers. And let's say B is a gene cluster that gives rise to individuals who are much more random about waking and sleeping times, are more aggressive, have a hard time concentrating on just one thing, and are very alert for novel stimuli. (We might even say that these individuals have ADHD or some other disorder.) A population of all type As might well be very productive, but it would also be very vulnerable to a nighttime raiding party of restless, marauding type Bs. A society of type Bs would be harder to sneak up on, but would be much less productive. And a society with 80% of A and 20% of B might beat both of the others with ease, overrunning the pure type A group and outproducing the pure type B group. But wait! What does "selection" mean in this case? Does it mean that the descendants of the mixed group will replace the pure groups? If the populations are so genetically separate that they can't interbreed, that's exactly what it would mean. But usually they aren't and it doesn't. Usually it means that a certain mix of characteristics will be selected for instead of one genotype or the other being "superior" or more "fit." This is no longer about the fitness of any one genotype, but rather about the fitness of a combination of genotypes. (Sounds like genes vs genotypes, right?) As circumstances change, maybe a 90/10 balance will become better than a 80/20. This will briefly and temporarily shift the selective advantage toward A, but it will be based on selection between groups, not within groups or between individuals. And what governs the changing balance is more likely to be either the evolution of a completely different set of genes, e.g., genes for mate selection, OR a difference in culture/learned behavior. And that leads us to... Cultural selection Technology, child-rearing methods, social organization, ideas... all of these make a difference in determining which groups succeed and grow and which decline and fade away. Again, this is not necessarily genetic selection. The winning culture can absorb the other individuals instead of destroying them. And the process is far more Lamarkian and chaotic than genetic evolution. Nevertheless, it exerts a strong selective effect on individuals, giving a reproductive advantage to those who adapt best to changing requirements. And a culture that has a superior technology for coping with its environment provides a major reproductive advantage to its members. Once again, however, the limit is felt when selection favors a culture in a way that alters its environment in a negative way. Irrigation allows feeding many more people, but irrigation without drainage eventually turns a fertile valley into a salt desert. Increased fitness at one level can be negated if it results in reduced fitness at another level. Ecosystem selection A single species, by evolving in a certain way, can change the evolutionary pressures on all the other species in its environment, gradually nudging them into other patterns. As the other species adapt, that changes the selective pressure on the first species and all the others as well. In a very broad sense, we know that this process tends to drive ecosystem evolution in the direction of increased efficiency in capturing energy and resources, and therefore in the direction of increased biomass. But in the shorter run, it can look much more like a drunkard's walk, with many surges and collapses. Founder effects can also be weirdly powerful. Sterilize a hundred identical plots of land and watch what happens. Those colonized by certain species first will go on to develop full ecologies that are often very different from the ones initially colonized by different species. So what does that do to the idea of natural selection and fitness? We tend to focus on the gene and the individual, but fitness only has meaning in a particular context: fitness to survive and reproduce in this location, this''population, ''this species, this ecology, this climate, this environment. And the paradox is that any sustained change in the genes and genotypes has the potential to trigger changes in the larger systems that give "fitness" its''meaning''. There is competition at every level, but it always comes with a limit: if "winning" at one level means "losing badly" at a different level, the selective advantage will be short lived.